We use an individual's fitness \(W\)  to measure the wellbeing (or performance) of an organism. Moreover, this is an instant value, defined for every time moment. Defining of the instant measure of an individual's performance is a highly non-trivial task. The term `fitness' is widely used in mathematical biology in essentially another sense based on the averaging of reproduction rate over a long time [1]}, [2]}, [3]}, [4]}. This is Darwinian fitness. It is non-local in time because it is the average reproduction coefficient in a series of generations and does not characterize an instant state of an individual organism.
